A conventional n-type transistor structure is known in which a layer made of Si:C or SiGe:C, etc., is formed on a region having B (boron) as a channel impurity diffused therein and a Si film into which an impurity is not implanted intentionally is formed thereon. This structure, for example, is disclosed in non-patent literary documents of Hong-Jyh Li et al., “Mat. Res. Soc. Symp. Proc.”, vol. 737, p. 643, 2003 and F. Ducroquet et al., “2004 IEDM Technical Digest.”, p. 437.
According to an n-type transistor described in the non-patent literary documents, since diffusion of B is suppressed in a Si:C layer, diffusion of B into the Si film is suppressed in a channel region, and it is thereby possible to form a channel region having a steep impurity concentration distribution.